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Ever since Edward Jenner first scratched cowpox pus into the arm of an eight-year-old boy in an attempt to render him immune to smallpox in May 1796, there has been debate and controversy over the procedure. Vaccination, which Jenner derived from the Latin word "vaca" meaning "cow," was preceded by the practice of "variolation" in which dried pus from an individual recovering from smallpox was scratched into the arm of someone naïve to the disease--usually a child. "Variolus" means "pus-like material."

Some astute individual in ancient China or India probably recognized that smallpox acquired by "variolation" was usually less deadly than acquired by other means. Jenner underwent variolation when he was an eight-year-old orphan. As an apprentice physician, he became aware of the local folklore that milkmaids who acquired cowpox infections developed immunity to smallpox. It was a logical step to substitute smallpox pus used in variolation with cowpox pus. Cowpox pus was more effective and less lethal in preventing smallpox than smallpox pus.

Unfortunately, there were problems. At the time, no one knew that they were dealing with a viral infection, so the cowpox pus was often contaminated with other microbes, including occasionally smallpox itself. James Gillray, a political cartoonist, published engravings showing cow heads sprouting out of people's bodies. People were fearful of receiving material originating from cows and opposed the procedure on religious grounds. (See the online Jenner Museum for more.)

But Jenner eventually prevailed, and approximately two centuries later, smallpox was successfully eradicated from human populations.

The discovery of the germ theory of disease and the field of immunology provided a scientific understanding of how vaccination works. Vaccines work by priming an individual's immune system to recognize and attack a pathogen (bacterial or viral) invader before it has a chance to sicken the person's body. Vaccines are usually weakened or killed pathogens, but they can also be parts of pathogens--the parts that elicit an immune response. Frequently, booster doses of vaccines are needed in order to get the body's immune system completely prepared. It's like training for battle at the microscopic level.

Louis Pasteur took vaccines to the next level. In 1880, he was working with the bacteria known to cause chicken cholera. He noticed that when he injected chickens with an old liquid cholera culture, they got sick briefly, but they didn't die. When he subsequently injected them with a fresh culture, the chickens remained healthy. He quickly saw the analogy with smallpox vaccination and understood that the concept of injecting weakened microbes to induce immunity could be applied on a wider scale. He succeeded in applying this principle to rabies by saving the life of a young boy who had been bitten by an infected dog. After that, people traveled far and wide, including the United States, to be saved by Pasteur's rabies vaccine.

Since then, vaccines have been developed for many diseases such as measles, diphtheria, and polio. In countries in which vaccination rates against the vaccine-preventable childhood diseases are high, epidemics rarely, if ever, occur. Outbreaks usually occur when unvaccinated individuals from countries with low vaccination rates visit.

Vaccination rates have to be high enough--around 90 percent-- so that "herd immunity" occurs. "Herd immunity" is the concept that if enough people are immune to a disease, then a chain of transmission cannot develop. Without it, susceptible individuals are likely to get exposed and sick.

However, it is important to point out that some individuals do have adverse reactions to vaccines, and that they aren't risk-free. The safety profile will increase when physicians can pre-screen individual's genetic profiles to predict how they will react. But we aren't there yet.

The latest vaccine controversy is the concern that vaccines containing thimerosal, particularly the measles, mumps, and rubella (MMR) vaccine cause autism. Thimerosal is a mercury-derived preservative used to prevent vaccine contamination. Paul Offit, director of the Children's Hospital of Philadelphia Vaccine Education Program, reviewed the studies both supporting and refuting the MMR vaccine-autism link [ "Vaccines and Autism"]. He found a number of flaws with the studies supporting the hypothesis, writing that the time an environmental insult would most likely lead to autism would be while the child is still in utero. A 2004 Institute of Medicine report also concluded that there was no link ("Immunization Safety Review: Vaccines and Autism").

Nevertheless, vaccine manufacturers have removed thimerosal from childhood vaccines to allay public concern. Still, the public and (in particular) the parents of the children with autism have not been appeased. Around 5,000 parents are suing the vaccine manufacturers because they believe that the vaccines caused their child's autism. If the plaintiffs win, it could have a profound impact on the availability of vaccines in the future.

Even without the lawsuit, the supply of vaccines has been dwindling. According to the 2003 Institute of Medicine report, "Financing Vaccines in the 21st Century," the number of vaccine manufacturers has decreased from 25 to 5 companies over the last 30 years. This trend developed because vaccines frequently generate lower revenues than drugs and not all insurance plans include them. In other words, the vaccine production and distribution infrastructure is crumbling, and shortages are occurring. If this lawsuit succeeds, there is a good chance that no one will remain in the vaccine business.

It has been easy to forget what life was like before vaccines. Untold numbers of people suffered and died from infectious diseases that can now be prevented. It would be a tragedy if we return to that era.